1. Field of the Invention
The present invention relates to solid-state image sensors and particularly to a solid-state image sensor included in an image pickup apparatus such as a digital camera.
2. Description of the Related Art
In recent years, there has been an increasing demand for images taken by apparatuses, such as a digital camera, with wider dynamic ranges. In response to such a demand, a solid-state image sensor is proposed by Japanese Patent Laid-Open No. 2004-363193, in which a plurality of photoelectric conversion portions having different areas are provided in each pixel, so that two kinds of pixel signals, which are a high-sensitivity signal and a low-sensitivity signal, are acquired. With a combination of the two signals, the dynamic range is widened.
There has been another increasing demand for a camera capable of taking a moving image and a still image simultaneously. In general, to acquire a smooth moving image, the moving image may be taken with a period of exposure time that substantially corresponds to the frame rate of the reading by a solid-state image sensor. In contrast, to take a still image, the period of exposure time may be set in accordance with the speed at which the object moves. Hence, to take a still image and a moving image simultaneously, two kinds of pixel signals based on different periods of exposure time need to be acquired.
In Japanese Patent Laid-Open No. 2004-120391, a solid-state image sensor is disclosed that includes a plurality of photoelectric conversion elements (equivalent to the photoelectric conversion portions according to Japanese Patent Laid-Open No. 2004-363193) provided in each pixel and being based on different periods of exposure time so that a moving-image signal and a still-image signal can be acquired simultaneously. Photoelectric conversion elements for a relatively short period of exposure time each have a relatively large area, whereas photoelectric conversion elements for a relatively long period of exposure time each have a relatively small area. Furthermore, the sensitivity of photoelectric conversion elements for a moving image is different from the sensitivity of photoelectric conversion elements for a still image.
Note that “sensitivity of a photoelectric conversion portion” is defined by the ratio of the amount of charge accumulated in the photoelectric conversion portion to the quantity of light that is incident on the pixel per unit time.
In each of the solid-state image sensors disclosed by Japanese Patent Laid-Open No. 2004-363193 and Japanese Patent Laid-Open No. 2004-120391, a desired image is taken with a plurality of photoelectric conversion portions that are provided in each of pixels and having different levels of sensitivity. In each of the devices, light is condensed by a microlens provided on the surface of the pixel, whereby the light is guided to each of the photoelectric conversion portions. Hence, the quantity of light that is incident on each of the photoelectric conversion portions varies with the angle of incidence of the light on the pixel. Therefore, the photoelectric conversion portions each receive only a portion of the light that is emitted from a specific part of the exit pupil of an image pickup lens used. Consequently, a blurred image of an object that is out of focus may be distorted, resulting in a deterioration of image quality.
The present invention is to suppress the deterioration of image quality by reducing the angular dependence of the sensitivity of each of a plurality of photoelectric conversion portions provided in each pixel and having different levels of sensitivity.